Implement SQLITEResultSet.first()

[vnayar]

If one attempts to run the following, a "not implemeneted" exception will be thrown:

auto statement = Connection.preparedStatement("Any SQL");
foreach (result; statement.executeQuery()) {
  ...
}

To avoid this, the unimplemented method "first()", which makes sure that the ResultSet is pointing at the first row, needs to be implemented. This is needed because foreach invokes ResultSetImpl.opApply(), which in turn invokes first().